The invention concerns a metallurgical vessel, in particular a converter for treating liquid molten metals, comprising a refractory lining and a supporting metal shell that surrounds the refractory lining.
Metallurgical vessels, such as converters or steelmaking ladles for example, are subjected not only to the static loading caused by the weight of the lining and the liquid molten metal with which they are filled but also to a thermal loading caused by the high temperatures. The supporting metal shell of these vessels must therefore be fabricated from a material which has a certain creep resistance, for example for 100,000 hours with respect to a predetermined temperature. The previously used steels, for example WStE.355, are capable of withstanding a maximum temperature of around 400xc2x0 C. Because of a high carbon content in the lining, accompanied by considerable burnoff of the lining toward the end of the run, the vessel shell temperatures rise.
In addition, the increasing secondary metallurgical treatment, for example vacuum treatment, which is frequently accompanied by not inconsiderable temperature losses, requires a higher melting temperature to compensate for this temperature loss. This results in greater thermal loading of the vessel, whether a converter or ladle, so that the requirement for adequate creep resistance at an elevated temperature becomes obligatory.
It is desireable that the steel be capable of withstanding a temperature of 500xc2x0 C., and preferably 550xc2x0 C. Creep-resistant steels which are suitable for this temperature range are known from the boiler tube sector, for example 15 Mo 3, 13 CrMo 44 or 10 CrMo 9.10. However, all these steels suffer from the great disadvantage that the component must be stress-relief annealed at a temperature of around 700xc2x0 C. after welding. This operation requires large furnaces and a large energy expenditure and the corresponding required handling. In particular in cases where repairs are made to vessels, the disadvantage mentioned occurs in particular. Repairs are necessary whenever the liquid molten metal has melted and partially destroyed the vessel shell because of excessive lining burnoff.
The object of the invention is to provide a metallurgical vessel of the generic type with a supporting metal shell having an adequate creep resistance up to max. 550xc2x0 C. which does not require stress-relief annealing after welding.
The above stated object is obtained by a metallurgical vessel of the invention.
The present invention is in a metallurgical vessel comprising a refractory lining having a supporting metal shell surrounding the refractory lining. The metal shell has welded shell rings and dished parts of creep-resistant steel with plate thickness of up to 100 mm. The creep-resistant steel is a highly resistant, water-quenched and then tempered close-grained structural steel having the following composition in wt.-%:
C 0.14-0.22
Cr 0.4-1.0
Mo 0.3-0.8
Ni 1.5-3.0
V 0.05-0.12
Mn 0.7-1.3
Pmax 0.015
Smax 0.003
Al 0.015-0.065
Si 0.20-0.60
Cumax 0.15
Nmax 0.012
Camax 0.004,
with the remainder being iron and impurities due to the production process.
The vessel of the invention displays a creep-resistance of 10,000 hours at 500xc2x0 C. of 220 N/mm2 and 550xc2x0 C. of 130 N/mm2. The vessel may be a liquid molten converter for steel.
Also included is a process for producing the vessel. That process includes a) melting a killed steel by the basic oxygen process with a composition corresponding to that set forth above; b) extruding a slab from the melted killed steel; c) heating the slab; d) rolling the slab into heavy plate; e) quenching and tempering the heavy plate; f) flame-cutting the plate into parts; g) bending and/or pressing into parts; h) welding the parts into a metal shell; i) stress-relief annealing; and j) fitting of the refractory lining into the shell. Prior to casting, the steel is calcium-treated by blowing a calcium alloy into the steel bath and is subsequently vacuum-treated. The hot rolling is carried out with a number of forming passes, which have an individual dimensional change of xcex5n greater than 0.1, in conjunction with quenching and tempering, thereby dispensing with stress-relief annealing after the welding of the parts produced from the heavy plate.
The quenching and tempering can be water-quenching from the austenitic range with subsequent tempering. The tempering temperature can be between 690 and 720xc2x0 C. The quenching and tempering can be heating up twice to the austenitic temperature with respect to water-quenching and tempering. It has been found that a highly resistant, water quenched and then tempered, close-grained structural steel known per se is well suited for this intended use. This close-grained structural steel is usually used with a yield strength of at least 890 N/mm2 in vehicle construction, for lifting apparatus, mining equipment and blower wheels. In these cases, the high yield strength is of particular significance so as to be able to use the lowest possible wall thicknesses and an adequate low temperature. See company brochure of Thyssen Stahl AG XAB090/XAB0960 Hochfeste vergxc3xctete Feinkornbaustxc3xa4hle [Highly resistant quenched and tempered close-grained structural steels (1993)].
Numerous tests with this close-grained structural steel have now shown that, by modifying the composition and by specific tempering treatment during quenching and tempering, one can obtain creep strengths which make its use for metallurgical vessels of interest. The yield strength lower than 890 N/mm2 is acceptable in this instance, since it is not the decisive criterion. Nevertheless, the still high yield strength of at least 650 N/mm2 still permits a certain reduction in the wall thickness, which is advantageous for the overall construction. A decisive advantage is, however, that no stress-relief annealing is required after welding. This results in a simplified production process and an energy savings. The increased nickel content of the composition permits full quenching and tempering even of plate thicknesses of up to 100 mm. Such wall thicknesses are sometimes required for the intended use. The known close-grained structural steel is usually designed for plate thicknesses of up to at most 50 mm.
The various features of novelty which characterize the invention are pointed out with particularity in the claims appended to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects obtained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.